Allow serialization of question state.

Summary:
This is needed by the SAT code to allow users to move forward and
backward through an exercise without losing their progress.

Test Plan:
Tested with my current SAT changes to allow forward and backward
navigation. Only tested with multiple choice questions at this time.

Reviewers: aria

Reviewed By: aria

Subscribers: dylan, #sat

Differential Revision: http://phabricator.khanacademy.org/D13740